Several years ago, using ab initio calculations, Melius et al. examined an interesting pathway for the homogeneous gas-phase water-gas shift reaction. They predicted that an enhancement in the rate of this reaction occurs at high water density due to changes in the generation of formic acid as an intermediate. To investigate this possibility, we have examined the conversion of CO and H2O to CO2 and H-2 in the absence of a catalyst in supercritical water at conditions from 410 to 520 degrees C and 2.0-60 MPa. In these experiments, Raman spectroscopy produced in situ, real-time measurements of reaction rates in an optically accessible, constant-volume reactor. The most rapid homogeneous rate measured, at 59 MPa and 450 degrees C, corresponds to an effective first-order reaction rate constant of 0.0033 s(-1). The data show that the measured reaction rates increase far faster than linearly as water density is raised above 0.35 g/cm(3), and the pressure dependence of the rate constant indicates that the transition state is characterized by an unusually high negative volume of activation, Delta upsilon double dagger = -1135 cm(3)/mol. These data provide strong evidence that this is not a simple gas-kinetic bimolecular reaction at conditions in the neighborhood of the critical density. Rather, the participation of extra water molecules in the structure of the transition-state complex results in a high-order rate dependence on water density. The data are analyzed within the context of a transition-state local density enhancement, expressed as a cluster number, xi(ts), found to be approximately 15 molecules.